Polyamides containing alkali metal halide additives as void formation inhibitors

ABSTRACT

POLYAMIDE COMPOSITIONS CONTAINING A SYNTHETIC POLYAMIDE INTIMATELY MIXED WITH 1.0 TO 10 PERCENT BY WEIGHT OF CERTAIN LITHIUM HALIDES ARE SPUN INTO LARGE DIAMETER, VOID-FREE, HIGH TENACITY MONOFILAMENTS WHICH ARE USEFUL, FOR EXAMPLE, IN ZIPPERS AND TIRE CORD.

United States Patent Office 3,591,565 Patented July 6, 1971 US. Cl. 26078 1 Claim ABSTRACT OF THE DISCLOSURE Polyamide compositions containing a synthetic polyamide intimately mixed with 1.0 to 10 percent by weight of certain lithium halides are spun into large diameter, void-free, high tenacity monofilaments which are useful, for example, in zippers and tire cord.

BACKGROUND OF THE INVENTION This application is a continuation-in-part of US. application Ser. No. 673,287, filed on Oct. 6, 1967.

Field of the invention This invention relates to synthetic linear polyamide compositions capable of being spun into void-free, large diameter monofilaments having a high tenacity and modulus.

Description of the prior art The synthetic linear polyamides prepared from polymerizable aliphatic and alicyclic dicarboxylic acids and aliphatic diamines possess a number of physical properties such as toughness and high tensile strength which make them of great value in many applications. The recurring intralinear carboamide groups in the said polyamides are separated by hydrocarbon groups containing at least two carbon atoms. Preparation and use of such polymers are illustrated in US. Pats. 2,071,250; 2,071,253; 2,130,948 and 3,393,210. The polyamides described in these patents are high molecular weight polymers which as a class are microcrystalline in structure. In general these polyamides have intrinsic viscosities above 0.4 where intrinsic viscosity is defined as in US. Pat. 2,130,948. They have recurring amide groups as an integral part of the main polymer chain, and are capable of being formed into filaments having structural elements oriented in the direction of the axis.

US. Pat. 3,272,773 discloses adding sodium iodide or other inorganic halides to various polycarbonamides in the amount of 0.1 to 5 percent by weight of the melt.

Heretofore, synthetic polyamide compositions, when spun into large diameter monofilaments, were found to contain voids which impaired those properties responsible for the strength and toughness of such monofilaments. The diameter of the monofilament that may be spun from a particular polycarbonamide without void formation is a characteristic of that particular polycarbonamide.

A void is an empty space in a filament in which polymer is not present. While it is generally recognized that all plastic filaments have microvoids which are not readily detected Without the aid of a microscope, it is also known that such filaments often contain macrovoids which are readily detected by the unaided eye. Void-free monofilamenets, therefore, as used herein refer to monofilaments which do not contain microvoids. Quantitatively, a monofilament is considered void-free if it contains no voids larger than 1 mil in diameter. Normally, void-free monofilaments of poly(hexamethylene adipamide) can be produced only when filament diameters do not exceed 25 mils after drawing the filament to 4 times its undrawn length. Frequently, significant void formation also results when large diameter monofilaments are spun from poly(hexamethylene sebacamide) and copolymers formed from hexamethylene adipamide and hexamethylene sebacamide, hexamethylene adipamide and epsilon-caproamide, and hexamethylene sebacamide and epsilon-caproarnide. The resulting voids make such monofilaments undesirable for use in zippers and tire cord where much dependence is placed upon the maximum retention of tensile strength and toughness.

In addition to the significant void formation which is found especially in large diameter monofilaments, it is known that frequent filaments breaks result from the high draw ratios required to produce high tenacity monofilaments. .Since voids provide a site where fracture of the monofilament can occur, it is believed that void formation may also be responsible for filament breaks.

SUMMARY OF THE INVENTION It has been discovered in accordance with this invention that certain lithium halide salts, when present to the extent of 1.0 to approximately 10 percent in synthetic polyamide compositions, have a pronounced effect on void formation when such compositions are spun into large diameter monofilaments.

In contrast with previous experience, it is now found that particular additives which control the crystallization rate of the polymer can be used, in the quantities herein disclosed, to control the formation of voids in polyamide compositions. In addition to reducing void formation, selected quantities of these additives were found to minimize the draw breaks resulting from the processing of monofilaments at the high draw ratios required to produce high tenacity filaments.

The preferred additives which are particularly effective in the practice of this invention are lithium chloride,

lithium bromide and lithium iodide. These salt additives are uniformly mixed throughout the molten polyamide resin at the time the resin is being extruded into monofilament. Normally such intimate mixing of salt and polymer is achieved by either adding the salt to the original charge of aqueous diamine and dibasic acid solutions or by coating the surface of the polyamide resin with salt crystals and subsequently extruding the resin.

In addition, the polyamides which are employed most efiectively in the practice of this invention are poly(hexamethylene adipamide), poly(hexamethylene sebacamide), poly(epsilon caproamide), the various copolymers of the foregoing and including epsilon-caprolactam as a comonomer, and poymers of bis (4-amino cyclohexyl) methane and decamethylne-l,IO-dicarboxylic acid.

DESCRIPTION OF THE PREFERRED EMBODIMENTS This invention is based upon the discovery that certain salts, when mixed with a polyamide in the requisite amounts, act as inhibitors to void formation in large diameter monofilaments. The salts used in the practice of this invention in order to be effective as inhibitors to void formation must:

(1) be soluble in the polyamide;

(2) be well mixed in polyamide;

(3) be thermally stable at the melt processing temperature;

(4) not contribute to the thermal decomposition of the polyamide.

Lithium chloride, lithium bromide and lithium iodide are examples of salts which effectively inhibit void formation in polyamides.

Normally, 1.0 to about 10 percent by weight of additive is mixed with the polyamide before a monofilament is formed. The upper limit on the quantity of additive used is dependent upon the additive used, the polyamide used, and the solubility of the particular additive in the particular polyamide under various process conditions. The preferred range of additive in polyamide, however, is 1.3 to percent by weight because excess additives may be extracted from the monofilaments by water and thereby impair the quality of the monofilament.

The polyamides which are employed most effectively in the practice of this invention are poly(hexamethylene adipamide), poly(hexamethylene sebacamide) copolymers of hexamethylene adipamide and hexamethylene sebacamide, hexamethylene adipamide and epsilon-caproamide, hexamethylene sebacamide and epsilon-caproamide, and the polymer of bis(4-aminocyclohexyl)methane and decamethylene-l,10-dicarboxylic acid.

Accordingly, the invention can best be illustrated with poly(hexamethylene adipamide) and lithium bromide; but it is to be understood that the procedures described below can be used equally well with other salts and polyamides.

The fact that void formation is minimized by the addition of lithium bromide is quite remarkable since void formation in polyamides is believed to result from volume changes which occur during quenching of the filament caused by a rapid crystallization at the surface of a filament as compared to the much slower crystallization occurring at the core of the filament. One would, therefore, normally expect a nucleating agent such as lithium bromide to increase rather than decrease void formations.

The addition of lithium bromide to poly(hexamethylene adipamide) also has the unexpected advantage of improving the processing behavior of the blended polyamide over unmodified polyamide. For example, a poly (hexamethylene adipamide) composition containing 2 percent by Weight of lithium bromide can be successfully spun and drawn into a 22 mil drawn filament exhibiting an unusually high tenacity and tensile modulus, e.g., tensile strength of 125,000 p.s.i. and tensile modulus of 900,000 p.s.i. An unmodified poly(hexamethylene adipamide) filament, however, was inoperable at the draw ratios (5.5 X to 6.5x) required to make such a filament because of the excessive number of draw breaks which occur.

Although various methods may be employed for mixing the salt and polyamide, the preferred methods are:

(1) Adding the salts to the aqueous charge of diamine and dibasic acid. The polymerization kettle used for such mixing, however, must be made of materials resistant to corrosion by the halides.

(2) Coating the salt crystals on the surface of the polyamide pellets by tumbling and then extruding the resin to adequately distribute the salt throughout the polyamide. When this procedure is employed it is often necessary to coat the resin pellets with a binder material prior to the addition of salt so as to insure an adequate adhesion of salt and resin. In addition, it is preferred that the salt be kept as dry as possible prior to the mixing of salt and polyamide in order to minimize the moisture content of the polyamide.

Once the salt additive and polyamide have been mixed, conventional spinning and drawing techniques are employed to form the monofilament. A spinning head with an appropriate round hole die is arranged for vertically spinning the filament into a quench tank. Quenching can then be achieved by contact with cool water. The quenching of molten polyamide after mixing is an important step in preparing the filament compositions of this invention, since it is expected that crystallization produces the undesirable void formation hereinafter described. After leaving the quench tank, the filament advances to a two stage system of draw rolls and draw heaters where the filament is stretched to its desired length prior to wind-up.

In addition to the processing problems described above, large diameter monofilaments often exhibited ovality rather than roundness. It has been found, however, that the roundess of some filaments can be greatly improved by using 38 C. quench water instead of 17 C. quench water. At higher quench temperatures the rate of crystallization in some filament proceeds more rapidly; and thus the filament is less likely to become oval. The use of higher quench temperature, therefore, makes it possible to produce large diameter, void-free monofilaments which remain relatively rounded.

EXAMPLES This invention is illustrated further by means of the following examples. The synthetic polyamides used in the practice of this invention are those of the nylon type having an intrinsic viscosity above 0.4 as defined in US Pat. 2,130,948.

EXAMPLE I Polyhexamethylene adipamide resin pellets were coated with 0.5 percent by weight of a binder composition, Santicizer 8 (a mixture of oand p-N-ethyl toluenesulfonamides, manufactured by Monsanto Company) which aids in binding salt crystals on the resin surface. The poly(hexamethylene adipamide)Santicizer 8 mixtures were tumbling in a container for three hours after which 1 percent by weight of lithium chloride which had previously been dried at 110 C. for 16 hours was added. This mixture was permitted to tumble for an additional three hours. The polyhexamethylene resin composition was then allowed to dry in a vacuum oven at 110 C. for 16 hours with a small bleed of nitrogen to keep out any air. This composition was extruded through a 1% inch extruder using a conventional screw and screen pack assembly (three-100 mesh stainless steel screen) and a die with a 90 mil orifice. The quench tank arrangement which was used had an air gap of 2 to 4 inches, path length of 18 inches and water temperature of 35 C. and a speed of 15 feet per minute.

For comparison purposes, poly(hexamethylene adipamide) resin pellets from the same lot were similarly extruded, but in the absence of the lithium chloride additive and Santicizer 8.

Under these standardized conditions, unmodified dry polyhexamethylene adipamide developed voids when the undrawn filament diameter was approximately 45 mils. The modified, lithium chloride containing composition, however, was spun into void-free undrawn monofilaments of approximately to mils in diameter.

EXAMPLE II The following example in which all parts are by weight unless otherwise specified, is illustrative of various embodiments of the present invention.

The following materials were added to a jacketed polymerization kettle in the following order with stirring:

Parts Distilled Water 900 Adipic acid 1000 hexamethylenediamine 1000 The mixture was stirred for /2 hour and the pH was adjusted to 7 by suitable addition of adipic acid or hexamethylene diamine. Approximately 2 percent by weight lithium bromide, based on the weight of polyamide composition, was added to the salt solution.

The salt solution containing lithium bromide was concentrated by vacuum distillation and transferred to a cylindrical glass bottle which was inserted into an autoclave having an inside diameter slightly greater than the outside diameter of the bottle. Polymerization was then carried out according to the following schedule:

The autoclave was cooled, and the plug of polymer in the bottle was removed by breaking the bottle. The plug of polymer was then crushed into smaller particles, dried in a vacuum oven at 110 C. for 16 hours and extruded through a 1% inch extruder coupled with a metering gear pump.

The extruded polyamide-lithium bromide mixture is then passed through a spinning head having a 150 mil diameter round hole die into a quench tank filled with water at C. The resultant 90 mil diameter monofilament was void-free.

Void-free monofilaments having undrawn diameters between and 200 mils were prepared from the above procedure through the use of different diameter round hole dies using higher percentages of lithium bromide for larger diameter monofilaments.

EXAMPLE III Example I was repeated using 1 percent by weight of a wide variety of additives. The following table summarizes the efiectiveness of various additives in minimizing void formation in undrawn poly(hexamethylene adipamide) monofilament.

TABLE I ADDITIVE CANDIDATES FOR VOID-FREE POLY HEXAMETHYLENE ADIPAMIDE) MONOFIL- AMENT Additive: Observation KCNS Thermal decomposition. LiCNS Thermal decomposition. LiI No voids at mil diameter. CaBr Voids at 50 mil diameter-not soluble. LiNO Thermal decomposition. Ca(CNS) Thermal decomposition. CsBr Voids at 55 mils diameter--not soluble. CsNO Thermal decomposition. CsCl Voids at 46 milsnot soluble. ZnBr Voids at 50-55 mils. CdCl Voids at 50-54 mils. Zn(NO Thermal decomposition. NaI Occasional voids at 63 mils. MgCl Thermal decomposition. NaBr Insoluble. NaCl Insoluble. CdBr Insoluble. MgBr Thermal decomposition.

EXAMPLE IV Various amounts of lithium halide salts were mixed with poly(hexamethylene adipamide) in an extruder and metering gear pump system under the following conditions of extrusion:

Melt temperature-290 C. Quench water temperature-17 C. Spinnerette hole diameter-150 mils Quenching time in bath-2 seconds Linear rate250 ft./ min.

The following table records a description of the monofilaments obtained in each instance.

TABLE IL-EFFECT OF LITHIUM HALIDE SALTS ON VOID FORMATION IN POLY(HEXAMETHYLENE ADIPAMIDE) MONOFILAMENT The effect of lithium bromide and quench temperature on void formation and roundness for large diameter monofilaments is shown in Table III.

TABLE III Filament size, Quench undrawn temperature N0. voids Composition (mils) C per 10 it.

Resin Without LiBr 55 25 100 Resin and 1% LiBr 72 X 77 43 0 Resin and 2% LiBr 53 X 65 17 0 Do 82 X 87 17 0 50 X 53 38 0 X 86 38 0 86 X 97 48 0 108 X 114 48 0 55 X 58 55 0 X 99 55 0 75 X 79 38 0 83 X 113 38 0 111 X 25 0 l Spinnerette hole diameter of 200 mils.

EXAMPLE V Example IV was repeated to establish the lower limit of lithium bromide content necessary to eifectively reduce void formation in poly(hexamethylene adipamide) monofialments. The monofilaments were treated in the normal fashion with the exception that the monofilaments were drawin to 4 times their undrawn length (draw ratio of 4 The voids were counted in 200 ft. of monofilament. The lithium bromide content was measured by ashing each sample at 500 C. (e.g., calculating the percent of ash in 50 gram samples). In addition, measurements on selected samples showed that the density of monofilament with voids was 3.5 percent lower than those with no voids.

The following table records a description of the monofilaments obtained in each instance.

TABLE IV.-EFFEGT OF LITHIUM BROMIDE ON VOID FO R- MAIION IN DRAWN POLY(HEXAMETHYLENE ADIPA- MIDE) MONFILAMENTS Filament size N0. voids Weight percent LiBr drawn (mils) per 200 ft.

The 30 to 40 mil diameter drawn monofilaments were useful for fabrication into zippers.

EXAMPLE VI through an external cooler. The drawing apparatus consisted of feed rolls, first and second stage draw rolls With typical skew roll stands, and first and second stage tubular 1% inch diameter furnaces with exposed electric resistance coils in ceramic blocks.

Three samples of 18 to 22 mil diameter drawn filaments (approximately 45 mils undrawn) were prepared from the poly(hexamethylene adipamide)1ithium bromide composition. Processing data for each group of samples are given below:

SAMPLES A AND B Melt temperature280 300 C.

Screw pump pressures:

ScreW600-80O p.s.i.g. Pump9001200 p.s.i.g.

Quench temperatures1518 C.

Drawing speeds 1st stage):

In50 ft./ min. Out225 ft./ min.

Drawing speeds (2nd stage) In225 ft./min. Out-265-280 ft./ min.

SAMPLE C Melt temperature280300 C. Screw pump pressures:

Screw-660860 p.s.i.g. Pump400750 p.s.i.g. Quench temperature-15-18 C. Drawing speeds (1st stage) In50 ft. min. Out210 ft./min. Drawing speeds (2nd stage) In210 ft./ min. Out-275 ft./ min. Total draw ratio=275/50=5.5

On addition, to spinning and drawing, the filaments in Sample C were steam treated with 110 p.s.i.g. steam to improve adhesion and to surface relax the filaments so that transverse properties such as loop and knot strength were improved.

The following table describes the properties of the monofilaments prepared by the above procedure.

methylene adipamide) in Examples IVI and are mixed with lithium chloride or iodide additives in a like manner with like results being obtained:

Examples: Composition VII-XII Poly(hexamethylene sebacamide). XIII-XVIII 80% hexamethylene adipamide/ 20% epsilon-caproamide copolymer. 10 XIX-XXIV 80% hexamethylene adipamide/ hexamethylene sebacamide copolymer.

EXAMPLE XXV A commercially available poly(epsilon caproamide) sold under the name of Plaskon 8200, containing about 1 percent caprolactam monomer mixed with 2 percent lithium bromide by tumbling pellets of the polymer with the lithium bromide for 2 hours. The pellets were then spun through an 180 mil orifice. No voids were present in the 125 mil products. When the poly(epsilon caproamide) without the addition of lithium bromide was spun, filaments 105 mils in diameter contained voids.

EXAMPLE XXVI The polymer of bis(4-aminocyclohexyl) methane and decamethylene-l,IO-dicarboxylic acid was prepared, in accordance with the teachings of US. Pat. 3,393,210. The polymer had a relative viscosity of 370, as measured in a 50 percent formic acid (98 percent concentration) 50 percent phenol solution at C., at a concentration of 3.7 grams of polymer per 50 mls. of solvent. Ninety percent of the diamino constituent of this high molecular weight polymer had the trans-trans stereoisomeric con- TABLE V.-EFFECT OF LITHIUM BROMIDE ADDITIVE ON HIGH TENACITY POLYHEXAMETHYLENE ADIPAMIDE HIGH DRAW' RATIOS MONOFILAMENTS PROCESSED A1 Total Filment Tensile draw diameter strength Modulus Elongation ratio (drawn) (p.s.i.) (p.s.i.) (percent) Observations 5. 5 17. 6-18. 0 1240, 00 850, 000 15. 5 N o draw breaks during 1% hours. Sample A... 5. 5 17. 7-18. 2 111'), 000 868, 000 14. 0 D0. 1 0. 0 18. 1-18. 3 121, 000 816, 000 14. 5 D0.

6. 0 17. 2-17. 4 138, 000 835, 000 16. 5 Do. 5. 5 21.2-21.2 124, 000 1, 000, 000 13. 5 Several draw breaks during 1 hour. sample 5. 5 20.1%21. 1 120, 000 922, 000 13. o No draw breaks during 1%1101115.

5.5 20.8-21.7 130,000 708,000 24.0 No draw breaks during 1% hours,

kngt stringtlli gas 42,0100 p.s.i. 1. 2. 126 000 838, 000 27.2 No raw rea 's uring 2 iours, sample 5 5 2 I 2 5 knot strength was 44,000 p.s.i.

5.5 21. 9-226 110,000 790,000 24.5 No draw breaks during 1% hours,

knot strength was 58,000 p.s.i.

1 Overhand knot.

The drawn 18 to 22 mil diameter monofilaments described above were void-free. Filaments prepared from unmodified poly(hexamethylene adipamide) processed under similar conditions were not void-free. In addition, frequent draw breaks were observed during the processing of the filaments prepared from unmodified poly(hexamethylene adipamide).

The high tenacity monofilaments described above were shown to be particularly useful as tire cord.

EXAMPLES VII-XXIV The following polyamide compositions are substituted individually in a like percent by weight for poly(hexafiguration. Samples of this polymer with and without 2 percent by Weight additions of lithium bromide or sodium iodide were extruded into large diameter monofilaments, by extrusion through a die having an aperture .250 inch in diameter. The conditions of extrusion were 295325 C. at pressures of 1000 to 1200 p.s.i. The filaments were quenched in an air gap, and then quenched in a commercial hydrocarbon (kerosene) bath at C. The commercial hydrocarbon bath is sold under the name Stoddard Solvent by Atlantic Richfield Company, and is known to consist of aromatic hydrocarbons, naphthenes and parafins. By increasing or decreasing the length of the air gap quench from 3 inches to 6.5 inches, larger or smaller diameter filaments were obtained. In order to uniformly disperse the lithium bromide or sodium iodide throughout the polymer, the polymer chips were first coated with 0.25 percent by weight benzenesulfonbutylamide and the polymer was then tumbled with the additive. Monofilaments of a diameter of 148-155 mils without the 2 percent addition of LiBr or NaI had voids. Monofilaments of a diameter of 175 mils containing 2 percent NaI had voids, while monofilaments containing 2 percent LiBr were void free up to the largest diameter tested230235 mils.

Results similar to that obtained with LiBr may be ob tained using LiCl and LiI.

As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that this invention is not limited to the specific embodiments thereof except as defined in the appended claim.

I claim:

1. A void-free monofilament of a composition consisting essentially of poly(hexamethylene adipamide) and 10 from 1.0 to 10% by weight of the composition of a member of the class consisting of lithium chloride, lithium bromide and lithium iodide.

References Cited UNITED STATES PATENTS 3,274,150 9/1966 Baevsky 260-459 OTHER REFERENCES Chemical Abstracts 62: 13326g, 65: 17114d, 66: 108659g.

ROBERT F. BURNETT, Primary Examiner US. Cl. X.R. 161172 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3 591 565 Dated July 6 1971 John Edward Hansen Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, line 73, "hereinafter" should read hereinabove Column 6, line 14, "do" should read Resin 3% LiF Column 7, Table V, "1240,00" should read l24,000 same Table V. last two items, after "strength", each occurrence, insert footnote l Signed and sealed this 14th day of March 1972.

(SEAL) Attest':

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents FORM PO-1U5O [10-69) USCOMM DC 6o376 p6g Q 0.5 GOVERNMENY PRHITING OFFICE was o-3ss-334 

